The outcry over the allegations that she shared information with a confidante, who does not hold any public office, led to a rare event in South Korean politics on Tuesday -- a televised presidential apology.
In it, Park admitted to sharing state documents with Choi Soon-sil, who gave "her personal opinion" on Park's speeches before the presidential election in 2012.
Park also said Choi looked at "some documents" for a certain period of time after Park took office, but didn't specify what they were.
"I am shocked and my heart is breaking for causing public concern," Park said in the live telecast. "I've done so (shared the documents) out of pure heart so that I could carefully review (the documents)".
The remark was Park's first since the scandal erupted this week.
Choi accused of intervening in state affairs
CNN South Korean affiliate JTBC broke the news of the scandal earlier this week after revealing that they had found an abandoned computer of Choi's containing evidence she received secret documents and intervened in state affairs.
Local media and opposition parties accused Choi of abusing her relationship with Park to force big local conglomerates to donate millions of dollars to two foundations they claim was she had set up.
Choi is also accused of pressuring a local university into giving her daughter special treatment, including changing school regulations so her daughter could get good grades without attending classes.
The chancellor of the university resigned last week, under mounting pressure from media and students.
A few thousand protesters from civil organizations and labor unions are expected to hold a demonstration on Saturday in the center of Seoul, according to South Korean police.
Local journalists and social media users have begun calling the scandal "Choi Soon-sil Gate".
Choi, who according to local media, is in Germany, was not reachable for comment.
South Korean Prosecutor's Office on Thursday established a "special investigation unit" to probe the cases.
While Park will not be subject to the investigation as per the country's criminal law, the prosecutor's office promised a thorough and quick investigation, according CNN affiliate YTN.
Park approval in freefall
The incident has hit Park's approval ratings. Her weekly approval rate plunged to a record low of 21.1% Thursday, according to a local pollster Real Meter.
Park, whose presidency ends in early 2018, enjoyed approval ratings in the 30 to 50% range during her first three years in office. This year has been a bad year for her, however.
A combination of a weak economy, inadequate public communications and poor administration of state affairs, according to multiple poll results, have resulted in slipping approval scores.
Park Geun-hye, South Korea's first female president, is the daughter of Park Chung-hee, South Korea's leader from 1961 to 1979, who was assassinated by his own intelligence chief. The late Park is hailed by some as the mastermind behind the country's current prosperity but criticized by others as a dictator who violated human rights and crushed dissent.